Fiction: 

The Tremendous Adventure of Major Brown - A wonderful story about a military man caught in a web of intrigue.

The Man Who was Thursday - A tale of anarchists, God, deception, and the meaning of life?

Manalive - A story about a man who is truly alive.

The Club of Queer Trades - Detective stories where nothing is as it seems

The Innocence of Father Brown - The Father Brown mysteries are among the best ever written

The Wisdom of Father Brown - More Father Brown

Tales of the Long Bow - More paradoxical tales

The Man Who Knew Too Much - Detective stories focusing on Horne Fisher

The Trees of Pride - An odd detective story filled with strange reversals